Add the `rate_limit` field to all response types,
which contains rate limiting information such as
maximum and remaining requests in the current
time frame, as well as when the next time frame
starts.
All response types now also implement the `ResponseType`
trait (internal for now). Currently this trait allows
converting from `ureq::Response` to the response type,
by extracting rate limiting information from the response
headers and deserializing the response body into the
response type.